dart2js: add builtin 'isSubtype'.

pkg/compiler/lib/src/js_emitter/old_emitter/emitter.dart

Index: pkg/compiler/lib/src/js_emitter/old_emitter/emitter.dart
diff --git a/pkg/compiler/lib/src/js_emitter/old_emitter/emitter.dart b/pkg/compiler/lib/src/js_emitter/old_emitter/emitter.dart
index 3a02c677e06608054777c32f0b44038b3c4f3172..bcc7977e3e670553270f27bc82353834e27090b5 100644
--- a/pkg/compiler/lib/src/js_emitter/old_emitter/emitter.dart
+++ b/pkg/compiler/lib/src/js_emitter/old_emitter/emitter.dart
@@ -299,6 +299,13 @@ class OldEmitter implements Emitter {
         return backend.rti.representationGenerator
             .templateForCreateFunctionType;
 
+      case JsBuiltin.isSubtype:
+        // TODO(floitsch): move this closer to where is-check properties are
+        // built.
+        String isPrefix = namer.operatorIsPrefix;
+        return jsAst.js.expressionTemplateFor(
+            "('$isPrefix' + #) in #.prototype");
+
       default:
         compiler.internalError(NO_LOCATION_SPANNABLE,
             "Unhandled Builtin: $builtin");